1. Field of the Invention
The present invention relates to a photopolyinerizable composition which can be cured by exposure to ultraviolet light, visible light and infrared light. More specifically, it relates to a photopolymerizable composition that is suitable for use as a recording layer in a negative planographic printing plate precursor.
2. Description of the Related Art
Various systems for making planographic printing plates directly from digital data of a computer have been developed. For example, photopolymerizable image-forming materials that are exposed to a laser emitting blue or green visible light have attracted much interest for use as a recording layer in a planographic printing plate that can achieve high printing resistance due to a tough coating cured by photopolymerization, because the photopolymerizable image-forming materials are sensitive to argon lasers or the equivalent and because it is possible to directly make plates that have high sensitivity using a photopolymerization initiation system.
One example of laser printing plates that utilize a photopolymerization initiation system sensitive to visible lasers such as argon lasers and that have been used are printing plates comprising an aluminum support having sequentially disposed thereon a photopolymerizable composition layer and an oxygen-blocking layer that inhibits polymerization. The photopolymerizable composition layer comprises a photopolymerization initiator and a compound including an addition-polymerizable ethylene double bond, and may further comprise if desired an organic high-molecular binder and a heat polymerization inhibitor. In these photopolymerizable planographic printing plates, an image is obtained by exposing a desired image, curing an exposed area by polymerization and removing (developing) an unexposed area with an alkaline aqueous solution.
In recent years, the development of lasers has been remarkable. In particular, high-output, compact solid-state lasers and semiconductor lasers that emit infrared light having a wavelength of 760 nm to 1,200 nm have become readily accessible. These lasers are quite suitable for use as a recording light source when plates are directly made from digital data of a computer. For this reason, in addition to the many photosensitive recording materials such as those mentioned above that are sensitive to visible light having a wavelength of less than 760 nm, materials recordable with these infrared lasers have been in demand. For example, photopolymerizable compositions comprising an infrared absorbent, a photopolymerization or heat polymerization initiator, and a compound having a functional group capable of being polymerized or crosslinked have been used as recording layers in negative image recording materials.
Photopolymerizable compositions that are cured by either visible light or infrared light basically comprise a polymerization initiator such as a radical generating agent, a monomer having a polymerizable functional group, and a binder polymer for improving a film property of the recording layer. A radical is generated from an initiator in the exposed (heated) region by exposure or heating, the radical causes a polymerization/crosslinking reaction of a polymerizable compound, and a curing reaction of a recording layer is conducted, whereby an image area is formed. The photopolymerizable composition contains a large amount of a polymerizable compound which is, in many cases, a relatively low-molecular compound such as a monomer or an oligomer.
Planographic printing plate precursors are usually stacked, stored, and transported. When the plate precursors are stacked, photosensitive layer surfaces and reverse surfaces of the supports are closely adhered. As a negative recording layer, the exposure-curable photopolymerizable composition in the photosensitive layer has a relatively low strength before being cured by crosslinking or polymerization, and contains a large amount of a polymerizable low-molecular compound. For this reason, the recording layer is relatively soft and plastic flow occurs easily.
There have thus been problems in that the support and the photosensitive layer easily adhere to one another and are difficult to separate when the plates are taken out one by one. Further, when plates deviate from the stack or are sequentially carried out for processing beginning with the uppermost plate in the stack, there have been problems such as the recording layer being deformed due to plastic flow and the surface of the recording layer sustains damage due to abrasion, whereby workability decreases and there is image trouble due to the recording layer sustaining damage during separation.